1. Field of the Invention
The present invention relates to mechanical fasteners and to electronic sensors for detecting failure of mechanical components, and particularly to a smart lid for smart bolts and probes that automatically detects failure of a smart bolt in a structure or machine and generates an alert for maintenance or emergency repair.
2. Description of the Related Art
Such structures as buildings, bridges, pipelines, petroleum plants, ships and aircraft must be robustly designed, carefully built and regularly inspected to prevent ‘wear and tear’ damage to components, such as bolts, from causing catastrophic failures. Even well designed and built structures will not last forever, so periodic inspection of bolts is often required to ensure structural integrity. As used herein, the terms “bolt” or “bolts” shall be taken to include any kind of bolt, including all tension-based bolts, screws, studs and fasteners.
Machines often have bolts with similar maintenance and inspection issues as structures. For example, failure of a bolt in a machine can cause delays and loss of productivity in both public and commercial enterprises. The maintenance of machinery constitutes a huge cost to industry. Some studies in Europe have shown the direct cost of maintenance often ranges between 4% to 8% of the total sales price. The indirect cost of maintenance issues, such as reduced quality and lost productivity, is likely to be at least that much. Unfortunately, bolts play a role in that in those maintenance issues and costs.
One aspect of dealing with the problem of bolt-related failure has been to ensure that the bolts were properly installed in the first place. When using bolted joints in industrial plants and mechanical systems, there often is some uncertainty regarding the desired amount of torque that should be applied, and how much corresponding preload is actually applied depends on the method of tightening selected, in part due to varying friction conditions. Insufficient preload, often caused by an inaccurate tightening method, is a frequent cause of bolted joint failure. While some design engineers appreciate the features and characteristics of the main methods employed to tighten bolts, whatever method is used to tighten a bolt, a degree of bolt preload scatter is to be expected, and hence a technique for failure monitoring of each bolt is desired. Furthermore, other applications include other types of failure criteria that also suggest local bolt monitoring.
One approach to dealing with preload variation and scatter has been the “smart bolt.” For example, smart bolts, such as those manufactured by Stress Indicators Inc. of Gaithersburg, Md. 20877 (accessible by Internet at www.smartbolts.com, marketed under the name SmartBolts®, provide a visual indication (a dot on the bolt head that changes color according to the condition of the bolt) of the tension that a bolt is under in a central portion of the bolt (“smart bolts”).
Some existing smart bolts are used in structures and industrial plants that have red dot, yellow dot, yellow dot then green dot, or other color combinations that turn black (or a different designated color) as the bolt is tightened. For example, the two-color (red/black) smart bolt from Stress Indicators Inc. will change from red to black as the bolt is sufficiently tightened. The three-color smart bolt from Stress Indicators Inc. will change from yellow to green as the bolt is optimally tightened, and then from green to black if the bolt is over-torqued. The color indicator in a smart bolt is also reversible, meaning that an optimally tightened bolt that becomes loose over time will change back to a color that indicates that it has not been sufficiently tightened.
Smart bolts provide an easily discernable visible indication that the smart bolt is properly tightened or not. This is even more important with some modern materials because internal damage in such materials can be very serious, but still leave little to no surface evidence, other than a smart bolt color indication. In all these cases, smart bolts provide a needed quick and accurate visual indication that something is wrong.
While smart bolts are a significant improvement over simple prior art bolts, structural and mechanical inspection by trained individuals, while necessary, is still both expensive and time-consuming. Even well-funded inspection programs can run into difficulties trying to inspect large numbers of widely scattered structures or machines because of the sheer magnitude of the task. For example, it is has been reported that there are roughly 600,000 bridges in the U.S. alone, and some tens of thousands are in need of repair or replacement. The sheer magnitude of the inspection task suggests mistakes in inspection have and will continue to occur.
Because the bolt inspection task is expensive, time-consuming, and subject to human error, it would be desirable to address these shortcomings. Thus, a smart lid for smart bolts and probes mitigating or solving the aforementioned problems is desired.